THE CHALK-RIVER AMS TIME-OF-FLIGHT DETECTION SYSTEM

At Chalk River, the final step in the identification of I-129 is a tim e-of-flight (TOF) measurement over a 2.5 m path. The start/stop signal s are derived from microchannel plates that detect accelerated seconda ry electrons emitted from thin carbon films. This system provides a re solution of 400 ps (FWHM) for the 50 MeV iodine ions and the TOF peaks are closely Gaussian. Resolution is limited by the uniformity of the start-detector foil. Masses 128 and 130 are separated from I-129 by ab out 2 ns, To improve transmission and to reduce energy straggling, the start detector has been constructed so that the beam is unobstructed by grids. In addition to TOF, the total energy (E) of the ions is meas ured with a passivated implanted planar silicon (PIPS) detector. Altho ugh I-129 standards and NaI blanks require only TOF for unambiguous id entification of I-129, many environmental samples are characterized by a plethora of peaks where both TOF and E are essential for unambiguou s identification of I-129.